1. Field of the Invention
This invention relates to surfaces on carbon-carbon composites, and more particularly to processes for creating specular surfaces on such composites.
2. Description of the Prior Art
Smooth coatings on carbon-carbon ("CC") composites are of utility in several fields, including fluids, structures and optics. For example, smooth coatings improve fluid flow across the surfaces of CC composites. Smooth coatings also provide CC composites with oxidation protection by providing a good foundation for subsequent oxidation protection coatings. Finally, optical emittance, absorptance, and reflectance are directly related to the smoothness or specularity of the surface.
Currently, the microcrack network inherent in CC composites yields at best a diffusely reflective surface, even after polishing. Such a surface is generally unacceptable for optical mirror applications.
CC composite optical mirrors are important components of many systems, but are especially important in space systems. The availability of a method for CC mirror fabrication is of particular interest because CC composites offer distinct advantages over conventional mirror materials, such as glass or metal. CC composites possess a high degree of dimensional stability, and therefore thermal shock resistance.
Dimensional stability is important for optical mirrors in order to preserve the shape of the optical surface. CC composites are also very light weight and strong, making them attractive for space applications. The only mirror materials with similar thermal expansion characteristics to CC composites, such as silica or low expansion glasses, are very heavy.
The prior art generally consists of applying carbon-forming lacquers directly to carbonaceous substrates. These lacquers generally consist of synthetic or natural polymers which have a high char yield in inert atmosphere. The lacquer is thermally decomposed in an inert atmosphere, generally to temperatures in excess of 800.degree. C., to yield a residual carbon coating on the substrate. During this decomposition (pyrolysis), the charing lacquer is extremely weak, allowing cracks and stresses, in the case of a CC composite substrate, to propogate through the carbon coating, as discussed above.
A variant of the above method consists of applying thermally decomposed carbon (pyrolytic carbon) to a substrate surface. This method also cannot form crack-free, specular surfaces on CC composites because of their anisotropic expansion. Localized thermal expansion anisotropies crack the pyrolytic carbon coating as the sample is cooled from the deposition temperature (&gt;850.degree. C.). Occasionally, it is possible to form very thick (&gt;0.5 mm) pyrolytic carbon layers which are largely or totally crack free, but this is neither very reproducible nor are the coatings thus formed very adherent. Such coatings are generally observed to preferentially spall off the substrate during intense mechanical or thermal loading. It is therefore a primary object of the present invention to provide a method by which a smooth, specular surface can be applied to a CC composite. It is also an object of this invention to provide a surface which can withstand extremes of heat and temperature, particularly those found in space.